Silicate-resin coating composition

ABSTRACT

A coating composition is disclosed comprising aqueous dispersions of copolymers, terpolymers, or ionomers of ethylene and ethylenically-unsaturated carboxylic acids or esters, blended with organic quaternary ammonium-silicate solutions. Coatings from these compositions are useful as electrical insulating coatings, particularly as applied to electrical steel sheets for magnetic cores, or to other metal sheets or wires, or as coatings or impregnants on other substrates. These organic/inorganic coatings have more heat resistance than purely organic coatings, and better adhesion, toughness, lubricity, and lack of abrasiveness than most inorganic coatings. When this coated steel is heated to hwgh temperatures for annealing, a tightly adhering coating of essentially inorganic silica is retained which has excellent electrical insulating properties.

United States Patent 191 Parkinson 1 Oct. 1, 1974 1 1 SILICATE-RESIN COATING COMPOSITION Robert E. Parkinson, Monroeville Borough, Pa.

[75] Inventor:

[73] Assignee: United States Steel Corporation, Pittsburgh, Pa.

[22] Filed: May 11, 1973 [21] Appl. No.: 359,372

[52] US. Cl. 260/29.6 WB, 117/1284, 117/135.l, 260/296 RW, 260/296 H [51] Int. Cl. C08f 29/04, C08f 45/24 [58] Field of Search..260/29.6 H, 2 9,6 WB, 4 48,2 N, 2 60/448. 8 A, 827, 29.6 RW; l17/135.1

[56] References Cited UNITED STATES PATENTS 3,239,549 3/1966 Weldes 260/4482 3,248,237 4/1966 Weldes et a1. 106/3835 3,383,386 5/1968 Weldes 260/2477 3,453,122 7/1969 Weldes et a1. 106/1 3,549,589 12/1970 Meincke 260/458 3,718,510 2/1973 Patula et al 148/14 OTHER PUBLICATIONS Weldes et al., Chemical Abstracts, 75: 120, 7802 (1971).

Weldes et al., Chemical Abstracts, 67: 65, 526d (1967).

Primary Examiner-Melvin Goldstein Assistant ExaminerW. Danison Attorney, Agent, or Firm-William L. Krayer [57] ABSTRACT A coating composition is disclosed comprising aqueous dispersions of copolymers, terpolymers, or ionomers of ethylene and ethylenically-unsaturated carboxylic acids or esters, blended with organic quaternary ammonium-silicate solutions.

18 Claims, No Drawings SILICATE-RESIN COATING COMPOSITION BACKGROUND OF THE INVENTION In the manufacture of grain-oriented electrical steel for use in magnetic cores of transformers or motors requiring the best possible magnetic properties, an inorganic coating such as Core Plate is usually applied as the final processing step. Such grain-oriented steel already has an oxide coating derived from the magnesia applied during the steel processing for protection during the anneal and its combination with iron and silicon oxides from the metal. The function of inorganic Core Plate 5 is to improve the electrical insulation and cover the sheet more uniformly with a more adherent and denser coating than that of the natural oxides. A common coating composition is a slurry of phosphoric acid and alumina, or aluminum phosphate, and inert fillers like mica, which is applied as a thin coating and baked to dry it and react it with the original oxides. This coating must retain its insulation properties after a stress-relief anneal of as high as 845C (1,550F) applied by the fabricator after slitting, punching, shearing, or winding the steel sheets. Sometimes nonoriented electrical steels are stress-relief annealed or otherwise heated by the fabricator and therefore have required an inorganic insulating coating. However, most non-oriented electrical steels do not require stress-relief annealing and are usually coated with an organic varnish (such as a phenol-formaldehyde-type called core plate 3, or an asphaltic-type called Core Plate 1) for the purpose of insulation, lubrication, and corrosion protection.

Some disadvantages of the various coatings of the prior art are that 1) some of them employ organic solvents, which are relatively difficult to handle and create a pollution problem, (2) sticking during the annealing step, (3) the inorganic coatings are generally highly acid, corrosive and unstable in storage, (4) poor adhesion and cohesion causing spalling or dusting of the coating and (5) abrasiveness on the dies, punches or shears, and (6) they usually must be chosen for a specific job.

Although certain quaternary ammonium silicates have been used in preparing coatings of certain types, and although certain ethylene copolymers have been used to coat the various substrates, there has been no reason to combine them in the manner which I propose to achieve an electrical insulating, protective and lubricating coating.

SUMMARY OF THE INVENTION The coatings applied from the compositions that I have developed can be made to meet the requirements of coatings for electrical steel and have certain substantial advantages over some of the materials now used.

a. They are water-based and therefore do not have the flammability and pollution problems of organic solvent-based varnishes.

b. The organic and inorganic components of these compositions can be balanced to give coatings with the lubricity, toughness and adhesion of the organic resins combined with the ability to leave a satisfactory high silicate residue coating with good insulation properties after most of the organic components are burned out during an anneal.

c. The handling of these compositions is more convenient than that of the usual inorganic coatings applied as highly acid slurries that are corrosive and unstable in storage. These new compositions may be low viscosity with high solids, and easily applied as thin coatings by conventional methods such as rolls, doctor-blades, sprays or clips.

(I. The fact that a single composition of my new material may be applied to electrical steel of all types, replacing either the usual organic (Core Plate 3) or inorganic (Core Plate 5) type of coating, simplifies the production handling and facilities required.

My coating compositions consist of aqueous solutions of organic ammonium silicates, such as tetraethanol or methyl-'triethanol ammonium silicate (Quram 4D or 220 made by Philadelphia Quartz Co.) blended with organic resins, preferably in the form of dispersions or solutions, such as ethylene-acrylic acid copolymer solutions (e.g., EAA 9300 or 9500 ammonia dispersions from Union Carbide Corp.) or ethylene copolymer, terpolymer or ionomer dispersions (e.g., Elvax D dispersions from E. l. duPont de Nemours Co.). The organic ammonium silicates I use differ from ordinary inorganic alkali silicates in that the quaternary ammonium cation is volatile and can be driven off by heating at moderate temperatures leaving essentially a silica residue.

The organic quaternary ammonium silicates employed are described generally in U.S. Pat. Nos. 2,689,245, 3,239,521, 3,239,549 and 3,248,237, and are preferably of the general formula WM O.X(N,,R,,. O.Y SiO .Z H O where W is a number from 0 to l, X is a number from 0.33 to 1.5, Y is a number from 2 to 10, Z is a number from 1 to 20, M is an alkali metal, R is an organic radical that forms an NR base selected from alkyl, alkanol, and heterocyclic groups, each independently selected, having from 1 to 20 carbon atoms, n is an integer from 1 to 10, and p is at least 4.

Typical examples of such materials are methyltriethanol or tetraethanol-ammonium silicates (Quram" from Philadelphia Quartz Company). These are commercially available as low-viscosity aqueous solutions with mole ratio of siO /organic cation of 0.5 to 22.5 with silica weight up to 45 percent. They are also available as dry powders of mole ratio of 7.5 and higher, with up to percent silica. The dry powder would have to be dissolved in water for use in my composition; it is generally more convenient to use an aqueous solution such as Quram 220 with a mole ratio of 12.5 SiO /organic cation.

I have found that it is possible to substitute a separate alkali metal silicate solution (such as sodium silicate with a SiO /Na O ratios of about 1.60 to 3.75) for up to about percent of the original organic-ammonium silicate of the above general formula. Stated another way, my composition may include soluble alkali metal silicate in an amount up to about three times the amount of quaternary ammonium silicate.

The ethylene copolymers, terpolymers and ionomers are thermoplastics having many of the properties of polyethylene, but because the carboxyl and hydroxyl groups on their chains, have excellent adhesion to metal surfaces and can be reacted or cross-linked under certain conditions. The term ionomer as used throughout is intended to mean a copolymer of ethylene and a monomer, such as acrylic, methacrylic, or crotonic acid, in the salt form, and which may be crosslinked by ionic bonds. The term neutralized or partly neutralized polymer of ethylene and carboxylic acid containing monomer is intended to include such ionomers. The preferred ethylene-acrylic acid copolymers are those with high acid content (e.g., 18 to 24 percent), such as Union Carbide Corporations EAA 9300 or EAA 9500, which can be dissolved as fluid, soap-like solutions or dispersions by heating with ammonium hydroxide or other alkali. Neutralization with volatile alkalis such as aqueous ammonia or amines is preferred for these coatings because the volatile alkalis may be driven off by heat, leaving a coating of the waterinsoluble original resin. These copolymer dispersions for use in my composition are prepared by placing the copolymer resin in a closed vessel with preferably an amount more than aqueous ammonia stoichiometric and heating to 90C to 130C with stirring until dissolved.

Preferred compositions of my invention are preferably prepared by forming a solution or dispersion of a quaternary ammonium silicate in a previously formed solution or dispersion of a copolymer of ethylene and a carboxylic acid containing monomer wherein the ethylene portion comprises about 60 to 90 weight percent, theethylene polymer being in a solution of a volatile alkali.

Aqueous dispersions of other copolymers, terpolymers, or ionomers of ethylene with other ethylenicallyunsaturated carboxylic acids (such as methacrylic or crotonic acids) or their esters, or with ethylenicallyunsaturated esters, such as vinyl-acetate, may be substituted for all or part of the ethylene-acrylic acid copolymer solutions in volatile alkalis. The ethylene/vinyl acetate copolymers I use have from 60 to 90 weight percent ethylene and are available as colloidal dispersions with 42 to 50 percent total solids; however, any convenient concentration may be used. Typical examples of such resin dispersions are Elvax D dispersions from E. l. duPont de Nemours, Inc. Since these resins are usually not alkali soluble, their dispersions are generally prepared by emulsion polymerization and are stabilized by surfactants. Such resin dispersions may contribute stability and lower cost to my composition, but it is generally preferred to retain a substantial portion of ethylene-acrylic acid copolymer dispersion because it appears to cross-link and develop better hightemperature properties in coatings from my compositions, and also to assist in dispersion of minor additives such as fillers or oils. 1

Other additions to my composition may include stabilizing amines, such as monoethanolamine or morpholine, which are effective in stabilizing the mixed dispersions to prevent gelling. My compositions are excellent media for the suspension of fillers, such as clays, talc, or mica, that may be desirable to improve the properties or reduce the cost of the coating. Pigments such as iron oxide or titania may be added for color or opacity. Surfactants may be added to provide better wetting or dispersion or to prevent foaming. Lubricants such as oils may be emulsified in the compound to improve lubricity or wear and abrasion resistance of the coatings. Thickeners may be added to control the viscosity of the dispersion. Other resins or resin-forming materials such as diacetone acrylamide or hexamethoxymethyl melamine or phenol-formaldehyde may be added to assist in cross-linking the ethylene polymers.

I have found that it is possible to blend these ethylene polymer dispersions and solutions with the organicammonium silicate solutions to give a fluid mixture that may be applied as a thin film to various substrates and then dried to give tough smooth adherent coatings. They may be prepared in weight ratios of SiO (derived from the quaternary ammonium silicate) to polymer of from about 20:1 to 1:20, but preferably are prepared and used in weight ratios of from about 1:1 to 10:1 of SiO to polymer.

Baking at 150C for several minutes or as high as about 400C for a few seconds gave satisfactory drying results. Coatings from these compositions are excellent as electrical insulating coatings, particularly as applied to electrical steel sheets for magnetic cores in coating thicknesses of about 1 to 5 microns (0.04 to 0.2 mils). The organic phase of the coating reduces the abrasiveness of the coating and gives more toughness or flexibility than the usual inorganic coatings for electrical steel. When these coated electrical steels were annealed at temperatures up to about 843C, most of the organics were burned out leaving a tightly adhering coating of essentially silica darkened with some residual carbonaceous material. This annealed coating retained excellent electrical insulation properties and there was no carburizing or other deleterious effect on the electrical steel.

These compositions may also be used as coatings on substrates and other metals where their heat resistance (compared to most resin coatings) toughness, adhesion, lubricity and lack of abrasiveness (compared to purely inorganic coatings), or chemical resistance, wear resistance, and corrosion protection would be desirable. As coatings or impregnants for paper, wood and other porous materials, they may impart resistance to moisture, oils, and heat, or improve the strength or wear resistance of the material. Because of their excellent electrical and heat resistance, adhesion, and toughness, they are useful as coatings for wire.

Table I gives examples of formulae which 1 have found to be satisfactory, as related in the following examples.

Table 1 Typical Formulae of Coating Compositions (parts by weight) Vinylacctatc Copolymcr* TABLE I Continued Typical Formulae of Coating Compositions (parts by weight) Example No.

Dispersion of Ethylene lonomer** l00 Silicate (N Silicate Misc. Additives Monoethanolamine 2-4 Morpholine Kaolin Clay Filler Lubricant (mineral oil or fat) Surfactants LII EAA 9300 or EAA 9500 (Union Carbide Corporation) Elvax lll2 Copolymer Dispersion or Elvax D1249 or D127] lonomer Dispersions (l-I.l. duPont de Nemours. Inc.) These ionomers are partially-neutralized ethylene/'methacrylic acid copolymers.

"" N-Silicate (Philadelphia Quartz Company) Examples 1 and 2 In Examples 1 and 2, ethylene-acrylic acid copolymer (about percent derived from acrylic acid), as the only organic resin, was blended as a 22 percent resin solids solution in aqueous ammonia with organic ammonium silicate solutions in the different proportions shown in the table. The addition of monoethanolamine as a stabilizer to the ethylene-acrylic acid dispersion before mixing into the silicate was found to be useful to prevent gelling. Morpholine was found to be effective in preventing gelling (but gave a higher viscosity) in Example 2, but gelling occurred rapidly in the higher organic/lower silicate, Example 1.

Stability of the mixture is also limited by the proportions of ethylene-acrylic acid resin to the silicate the high silicate Example 2, was stable for several months, while Example 1 increased in viscosity so that it would be unsatisfactory for use without dilution in about two weeks. Higher EAA proportions gelled rapidly. Higher melt index EAA copolymers were more stable in these examples than low melt index EAA copolymers.

The coatings from Examples 1 and 2 roll-coated an oriented and non-oriented electrical steels were baked at 300C for l to 5 minutes, giving dry film thicknesses of about one to 2.5 microns (0.04 to 0.10 mils). Example l coatings were smoother and thicker than Example 2. When coated oriented steel was stress-relief annealed at about 800 to 850C (1,475 to l,550F) in a reducing atmosphere, Example 1 coatings were glossy, black, and very adherent while Example 2 coatings were duller gray and somewhat less adherent. Both of these coatings had no adverse effect on magnetic properties by the Epstein test (ASTM A343) and insulation of these very thin films was good, before and after annealing, by the Franklin test (ASTM A344).

Examples 3 and 4 In these examples, the dispersions of ethylene-vinyl acetate copolymers or ionomers were blended with the quaternary ammonium silicate. The mixtures were found to be very stable over a wide range of proportions. It was not necessary to add monoethanolamine or other amine as a stabilizing agent, as it was with ethylene-acrylic acid copolymers. These compounds were applied to electrical steel in the same manner as those of Examples 1 and 2, and the coatings were found to have equivalent properties.

Example 5 This composition contained ethylene-acrylic acid copolymers and ethylene-ionomer dispersions blended with the organic quaternary ammonium silicate. This formula was preferred to the previous examples because it was more stable and gave smooth, uniform, adherent, abrasion-resistant coatings with excellent insulation properties. It was applied to electrical steels by roll-coating in a concentration of about 38 to 40 percent solids, and baked l to 3 minutes in an oven at 300C or 30 seconds at about 400C, giving smooth adherent coatings with excellent electrical insulation as may be seen from the following data. When these coated steel sheets were annealed at about 845C, a smooth, dark adherent coating remained with excellent insulation properties (0.00 to 0.10 amperes by the Franklin Test, ASTMA 344.68, at C, 300 psi).

Example 6 In this example, sodium silicate was substituted for a portion of the organic ammonium silicate and a kaolin clay filler was added in order to reduce the cost of the composition. It was found to be desirable to emulsify a small amount of lubricating oil or fat in the composition to improve the resistance of the coating to abrasion or wear when coated sheets were rubbed together. When applied and baked as in Example 5, the coating results were similar except that they were less glossy.

I do not intend to be restricted to the above specific illustrations and examples of my invention. It may be mer component is a mixture of 7 otherwise variously practiced within the scope of the following claims I I claim: 1; Composition useful for coating substrates comprising a solution or dispersion in an aqueous'm'edium of -a. "quaternary ammonium silicate of the formula number from O to 1, X is a number from 0.33 to 1.5, Y is a number from 2 to 10, Z is a number from 1 to 20, M is an alkali metal, R is an organic radical that forms an NR base selected from alkyl, alkanol, and heterocyclic groups, each independently selected, having from 1 to 20 carbon atoms, -n is an integer froml to 10, and p is at lsest: d

. ethylene polymer selected from the group consisting ofcopolymers, terpolymers and ionomers of ethylene with acrylic, methacrylic or crotonic acids or esters thereof or vinyl acetate, wherein the portion of ethylene polymer derivedfrom ethylene is from about 60 to about 90 weight percent, the weight ratio of SiO derived from the quaternary ammonium silicate to ethylene polymer being from about 20:1 to about 1:20. 1 r

2. Composition of claim ly in which the quaternary ammonium silicate is in the form of an aqueous solution and theethylene polymer is in the form of an aqueous solution, dispersion or latex, Y

3. Composition of claim 1 in which the ethylene polymer is an ammonium, alkali metal or volatile alkali ionomer.

4. Composition of claim 5. Composition of claim 1 in which (NHR is (C 2 H2 )i.

6. Composition of claim 1 in which the weight ratio 1 in which (N,,R,,) is'CH of SiO to ethylene polymer is from about 1:1 to about 7. Composition of claim 1 in which the ethylene polymer is a copolymer of ethylene and vinyl acetate and is in the form of an aqueous dispersion or latex.

8. A composition of claim '1 in which the ethylene polymer is a copolymer of ethylene and acrylic acid and is in the form of an aqueous ammonium'salt solution or, dispersion.

9. Composition of claim 1 in which the ethylene polya copolymer of ethylene and acrylic acid in the form of an aqueous ammonium salt solution and 2. an ionomer of ethylene and metacrylic acid in the formof an aqueous dispersion.

10. Composition of claim 1 including soluble alkalimetal silicate in an amount up to about three times the amount of quaternary ammonium silicate.

11 Method of making acornposition useful in forming coatings comprising forming a solution or dispersion of a quaternary ammonium silicate'of the formula WM O.X(N,,R,,) O.Y S iO .Z H O where-W is a number from O to 1,X is a number from 0.33 to 1.5, Y is a number from 2to' 10, Z is a number from 1' to 20, M is an alkali metal, R is an organic radical that forms an NR base selected from alkyl, alkanol, and heterocyclic groups, each independently selected, having from 1 to 20 carbon atoms, n is an integer from 1 to 10, and p is atleast 4', in a previouslyformed solution or dispersion of a copolymer of ethylene andan ethylenically unsaturated carboxylic acid containing monomer wherein the ethylene portion comprises about 60,to weight percent, said ethylene polymer-being in a solution of a volatile alkali.

12. Method of claim 11 in which the volatile alkali is ammonium hydroxide. v

13. Method of making a composition usefulfor coatingsubstrates comprising a. dissolving a copolymer of ethylene and acrylic acid in an aqueous solutionv of volatile alkali at a temperature from about90C to about C, and

b. mixing into the solution thus formed a quaternary ammonium silicate solution to yeild a composition with a weight ratio of Si0 derived from the quaternary ammonium silicate to ethylene copolymer solids of from 1:20 to 20:1.

14. Method of claim 13 in which the solution of volatile alkali is aqueous ammonia.

15. Method of claim 13 in which the ethylene copolymer is a copolymer of ethylene and from about 10 per cent to 30 percent acrylic acid.

16. Composition of claim 1 stabilized against gelling including an effective amount of a stabilizing amine.

l7. Composition of claim 1 in which the ethylene polymer component is a mixture of a. a copolymer of ethylene and acrylic acid in the form of an aqueous ammonium salt solution; and

b. a copolymer of ethylene and vinyl acetatein the form of an aqueous dispersion.

l8. Composition of claim 1 in which the ethylene polymer component is a mixture of a. a copolymer of ethyleneand acrylic acid in the form of an aqueous ammonium salt solution; and

b. a terpolymer of ethylene, vinyl acetate and methacrylic acid in the ,formof an aqueous dispersion. 

1. COMPOSITION USEFUL FOR COATING SUBSTRATES COMPRISING A SOLUTION OR DISPERSION IN AN AQUEOUS MEDIUM OF A. QUANTERNARY AMMONIUM SILICATE OF THE FORMULA WM2O.X(NNRP)2O.Y SIO8Z H2O WHERE W IS A NUMBER FROM 0 TO 1 X IS A NUMBER FROM 0.33 TO 1.5, Y IS A NUMBER FROM 2 TO 10, Z IS A NUMBER FROM 1 TO 20 M IS AN ALKALI METAL, R IS AN ORGANIC RADICAL THAT FORMS AN NR BASE SELECTED FROM ALKYL, ALKANOL, AND HETEROCYCLIC GROUPS, EACH INDEPENDENTLY SELECTED, HAVING FROM 1 TO 20 CARBON ATOMS, N IS AN INTEGER FROM 1 TO 10, AND P IS AT LEAST 4, AND B, ETHYLENE POLYMER SELECTED FROM THE GROUP CONSISTING OF COPOLYMERS, TERPOLYMERS AND IONOMERS OF ETHYLENE WITH ACRYLIC, METHACRYLIC OR CROTONIC ACIDS OR ESTERS THEREOF OR VINYL ACETATE, WHEREIN THE PORTION OF ETHYLENE POLYMER DERIVED FROM ETHYLENE IS FORM ABOUT 60 TO ABOUT 90 WEIGHT PERCENT, THE WEIGHT RATIO OF SIO2 DREIVED FROM THE QUATERNARY AMMONIUM SILICATE TO ETHYLENE POLYMER BEING FROM ABOUT 20:1 TO ABOUT 1:20.
 2. Composition of claim 1 in which the quaternary ammonium silicate is in the form of an aqueous solution and the ethylene polymer is in the form of an aqueous solution, dispersion or latex.
 2. an ionomer of ethylene and metacrylic acid in the form of an aqueous dispersion.
 3. Composition of claim 1 in which the ethylene polymer is an ammonium, alkali metal or volatile alkali ionomer.
 4. Composition of claim 1 in which (NnRp) is CH3 N(CH2CH2OH)3.
 5. Composition of claim 1 in which (NnRp) is N(CH2CH2OH)4.
 6. Composition of claim 1 in which the weight ratio of SiO2 to ethylene polymer is from about 1:1 to about 10:1.
 7. Composition of claim 1 in which the ethylene polymer is a copolymer of ethylene and vinyl acetate and is in the form of an aqueous dispersion or latex.
 8. A composition of claim 1 in which the ethylene polymer is a copolymer of ethylene and acrylic acid and is in the form of an aqueous ammonium salt solution or dispersion.
 9. Composition of claim 1 in which the ethylene polymer component is a mixture of
 10. Composition of claim 1 including soluble alkali-metal silicate in an amount up to about three times the amount of quaternary ammonium silicate.
 11. Method of making a composition useful in forming coatings comprising forming a solution or dispersion of a quaternary ammonium silicate of the formula WM2O.X(NnRp)2O.Y SiO2.Z H2O where W is a number from 0 to 1, X is a number from 0.33 to 1.5, Y is a number from 2 to 10, Z is a number from 1 to 20, M is an alkali metal, R is an organic radical that forms an NR base selected from alkyl, alkanol, and heterocyclic groups, each independently selected, having from 1 to 20 carbon atoms, n is an integer from 1 to 10, and p is at least 4, in a previously formed solution or dispersion of a copolymer of ethylene and an ethylenically unsaturated carboxylic acid containing monomer wherein the ethylene portion comprises about 60 to 90 weight percent, said ethylene polymer being in a solution of a volatile alkali.
 12. Method of claim 11 in which the volatile alkali is ammonium hydroxide.
 13. Method of making a compOsition useful for coating substrates comprising a. dissolving a copolymer of ethylene and acrylic acid in an aqueous solution of volatile alkali at a temperature from about 90*C to about 130*C, and b. mixing into the solution thus formed a quaternary ammonium silicate solution to yeild a composition with a weight ratio of SiO2 derived from the quaternary ammonium silicate to ethylene copolymer solids of from 1:20 to 20:1.
 14. Method of claim 13 in which the solution of volatile alkali is aqueous ammonia.
 15. Method of claim 13 in which the ethylene copolymer is a copolymer of ethylene and from about 10 percent to 30 percent acrylic acid.
 16. Composition of claim 1 stabilized against gelling including an effective amount of a stabilizing amine.
 17. Composition of claim 1 in which the ethylene polymer component is a mixture of a. a copolymer of ethylene and acrylic acid in the form of an aqueous ammonium salt solution; and b. a copolymer of ethylene and vinyl acetate in the form of an aqueous dispersion.
 18. Composition of claim 1 in which the ethylene polymer component is a mixture of a. a copolymer of ethylene and acrylic acid in the form of an aqueous ammonium salt solution; and b. a terpolymer of ethylene, vinyl acetate and methacrylic acid in the form of an aqueous dispersion.
 20. 